1. Field of the Invention
The present invention relates to a highly rigid propylene resin composition and to a blow-molded article made from the composition. More particularly, the invention relates to a highly rigid propylene resin composition which enables manufacture of large-sized blow-molded articles featuring light weight and excellent rigidity, dimensional stability, and heat resistance and which shows good drawdown resistance and is suited for continuous blow molding as well as to a blow-molded article which is made from the composition and is advantageously used as, particularly, an automobile bumper, bumper beam, or a like part.
2. Description of the Related Art
Conventionally, propylene resins are used as general-purpose resins for manufacturing products of various shapes by various molding methods, including extrusion molding, injection molding, and blow molding. Because blow molding makes use of inexpensive molds and is a simple molding process attained through employment of integral molding, it has been intensively used for molding large-sized structural elements, mainly automobile parts. In view of specific gravity, rigidity, dimensional stability, and heat resistance, in many cases propylene resins are used as materials for such blow molding. However, such conventionally used propylene resins are not necessarily satisfactory in terms of drawdown resistance or rigidity required for blow molding.
To solve the above problem, various improvements on propylene resins have been attempted. For example, U.S. Pat. No. 4,550,145 and U.S. Pat. No. 5,736,613 disclose a highly rigid propylene resin which is manufactured through multi-step polymerization, including polymerization of propylene and copolymerization of propylene and ethylene, and which allows manufacture of a large-sized blow-molded article featuring light weight and excellent rigidity, dimensional stability, and heat resistance, through adjustment of its melt index and the relation between its elongational viscosity and its melt index.
The thus-obtained propylene resin shows improved drawdown resistance in an initial stage of continuous production of large-sized blow-molded parts having a parison weight of, for example, not less than 3 kg, particularly not less than approximately 5 kg. However, as blow molding continues, the propylene resin becomes impaired in drawdown resistance, resulting in disabled molding or nonuniform thickness distribution of a blow-molded product. Thus, continuous production using the propylene resin as a blow-molding material tends to become unstable.
Accordingly, an object of the present invention is to provide a propylene resin composition which enables manufacture of large-sized blow-molded articles featuring light weight and excellent rigidity, dimensional stability, and heat resistance and which shows good drawdown resistance.
Another object of the present invention is to provide a blow-molded article which is made from the composition and is advantageously used as, particularly, a large-sized automobile part, such as a bumper, a bumper beam, a seat back, an instrument panel, or a like part.